The present invention relates to a multi-stage sprocket assembly for a bicycle and more particularly to a multi-stage sprocket assembly having chamfered sprockets configured to facilitate the shifting of a drive chain from a larger sprocket to a smaller sprocket.
According to a conventional multi-sprocket assembly, the large sprocket of the assembly includes a disengagement-facilitating means for facilitating disengagement of the drive chain from teeth of the large sprocket when the drive chain is shifted from the large sprocket to the small sprocket to change a driving speed of the bicycle. Specifically, the disengagement means comprises reduction in the height of some of the teeth of the large sprocket relative to the other teeth in order to facilitate the chain disengagement at the specified portion of the large sprocket.
Various features have been added to the traditional multi-sprocket assembly to ensure smooth and reliable chain shift action from the large sprocket to the small sprocket. For example, according to a conventional multi-stage sprocket assembly, the large sprocket has, in its face facing the small sprocket, a concavity to form a stepped portion for receiving the shifting drive chain when the chain is shifted from the large sprocket to the small sprocket disposed adjacent thereto. Specifically, the stepped portion is provided at a position higher than the dedendum of the teeth of the large sprocket so as to avoid locking of the shifting chain segment to the large sprocket.
The problems with the chain shift action is exacerbated by environmental conditions, such as mud and dirt, which can collect in the drive chain links and in the stepped portions of the sprocket assembly. FIG. 1a depicts a typical drive chain 10 having a plurality of links 12, wherein each link includes an inner plate 14 and an outer plate 16. As shown in FIG. 1b, when a typical new, clean drive chain 10 is bent, the drive chain has a bending arc 18 of approximately 300 mm. However, when there is shifting in mud or dirt, particles gather in the links of the drive chain. As a result of the fine mud and dirt particles settling into the links 12 of the drive chain 10, the drive chain becomes stiff and the bending arc 18 decreases in height. FIG. 1c depicts a drive chain that has been stiffened due to the settling of mud and dirt particles in the links 12 of the drive chain 10. As shown in FIG. 1c, the bending arc 18xe2x80x2 has decreased to 100 mm, significantly less than the bending arc 18 of a new, clean drive chain. When the drive chain is in a stiffened condition, the occurrence of chain jamming incidents significantly increases.
Accordingly, it is desirable to provide a multi-stage sprocket assembly for a bicycle that offers smooth and reliable chain shift action from the large sprocket to the small sprocket even in inclement conditions, such as mud or dirt.
The present invention overcomes the disadvantages of the prior art. A chamfered sprocket assembly for facilitating the shifting of a drive chain from a larger sprocket to a smaller sprocket is disclosed. In a preferred embodiment of the invention, the chamfered sprocket assembly includes a sprocket having a plurality of chamfered portions on a side face facing a smaller sprocket, each chamfered portion located between a pair of toothlike projections located on the rim of the sprocket body. Each of the chamfered portions preferably include a crest which tapers toward a first edge and a second edge of the chamfer portion, respectively. The crest is preferably offset from the center of the chamfer portion toward the driving direction of the sprocket assembly. If there are more than two sprockets in the sprocket assembly, the intermediate sprockets also have chamfered portions to facilitate the smooth transition of the drive chain from one sprocket to another.
To further facilitate the shifting of the drive chain, the sprocket preferably includes a rounded shoulder to guide the movement of the drive chain. Finally, the friction between the toothlike projections and the drive chain is reduced by polishing the toothlike projections.
Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.